Relax the bounds of dP_dx

watertap/core/membrane_channel0d.py

@@ -147,7 +147,7 @@ def _add_pressure_change(self, pressure_change_type=PressureChangeType.calculate
             self.dP_dx = Var(
                 self.flowsheet().config.time,
                 initialize=-5e4,
-                bounds=(-2e5, -1e3),
+                bounds=(-2e5, None),
                 domain=NegativeReals,
                 units=units_meta("pressure") * units_meta("length") ** -1,
                 doc="pressure drop per unit length across channel",